It is known that shut-off valves are used to switch the flow from a source of pressure into pressureless circulation, as long as a predetermined pressure-level is maintained in a hydraulic system. In order to extend the time between two switching points, hydraulic accumulators are usually incorporated into the hydraulic system. The hydraulic system and the accumulators are fed jointly from the source of pressure through a check-valve which prevents a return-flow of the pressure medium from the hydraulic system when the shut-off valve is switched to circulate. The pressure for controlling the pilot-valves is taken off in each case downstream of the said check-valve and thus corresponds at all times to the pressure in the hydraulic system.
Known shut-off valves of this type always function satisfactorily when the pressure-gradient, during charging of the hydraulic circuit, is steep or, in other words, when the time taken to reach the upper switching point is short. In the case of very small pressure-gradient, such as arise, for example, when the supply-flow is only slightly greater than the consumption, or when very large accumulator must be filled with small pumps, it frequently happens that the main control-piston switches-through and remains in an intermediate position producing a back-pressure. Another disadvantage occurs when a hydraulic system is to remain under pressure for a long period of time. Since the pilot-valves are subjected to the pressure of the hydraulic system, a certain amount of leakage is unavoidable and this leads to an immediate drop in pressure and unnecessary functioning of the shut-off valve. This behaviour is also unsatisfactory from the point of view of energy balance. As far as the general switching behavior of these shut-off valves is concerned, other disadvantages arise at times, in that the set switching pressures vary as a function of other operating parameters, presumably as a result of dynamic processes in the valves.